UGRINDING PRECISION AND ACCURACY OF FITOF CEREC 2 CAD-CIM INLAYSWERNER H. MORMANN, D.M.D., D.D.S., PH.D.; JENS SCHUG, D.D.S.

The authors conducted research

to determine the grinding preci-

sion and accuracy of fit of ce-

ramic inlays generated with the

completely redesigned CEREC 2

computer-assisted design/com-

puter-integrated manufacturing,

or CAD-CIM, unit. They found

that the grinding precision of the

CEREC 2 unit was 2.4 times

greater than that of CEREC 1.

With CEREC 2, the mean (+ stan-

dard deviation) luting interfaces

were 56 + 27 micrometers,

which is a 30 percent improve-

ment in the accuracy of fit com-

pared with that of CEREC 1.

Ohe CEREC (Siemens AG) computer-assisted design/computer-in-tegrated manufacturing, or CAD-CIM, method has been used in clin-ical dentistry for more than 10 years.' More than 3,000 practitionersworldwide are using the four versions of this system to generaterestorations in their practices. The advances ofCAD-CIM technologyhave been reported extensively in the dental literature.'-3 In vitro4and in vivo studies5 have examined the accuracy of fit and dimen-sions of luting interfaces of the CAD-CIM restorations.

The first clinical investigation of computer-generated ceramicrestorations was begun in 1986.5 The inlays were fabricated usingthe original CEREC 1 (a) hardware (BRAINS AG) and software(CEREC Operating System, or COS, version 1.0; BRAINS AG).67Examination by scanning electron microscopy, or SEM, showed oc-clusal luting interfacial widths to be 140 to 265 micrometers.5

Using the second-generation CEREC 1 (b) unit developed in 1988(Siemens AG) and COS 2.0 software developed in 1991 (SiemensAG), researchers clinically measured interfacial widths, which werea mean (± standard deviation) of 169 ± 48 pm.8 In an in vitrostudy, Inokoshi and colleagues9 measured the marginal interfacesin Class II CEREC restorations using second-generation CEREC 1(b) hardware and COS 2.1 software; they found that widths rangedbetween 50 and 99 pm.

The third-generation CEREC 1 (c) unit was introduced in 1992.10An electric motor significantly increased the engine power as wellas the service life of the grinding disks." The higher rigidity of thesystem and a smaller diamond grain diameter (64 pm) of the grind-ing wheel considerably improved the marginal integrity of the ma-chined restorations." The precision of the margins was reported tobe in the 80- to 120-pm range.'2

The completely redesigned CEREC system, called CEREC 2, isable to produce inlays, overlays, veneers and full crowns. Throughimprovements in the grinding process and optimized three-dimen-sional, or 3-D, scanning, it aims to achieve an even higher accuracyof fit than previous systems achieved.'3 The further development ofthe intraoral 3-D camera was done in accordance with the originalCEREC process.6

JADA, Vol. 128, January 1997 47

R ESEARCH-

Figure 1. The monitor of the CEREC 2 computer-as-sisted design/computer-integrated manufacturingunit (Siemens AG) shows an intraoral optical im-pression of a mesio-occlusodistal cavity in a lowerleft first molar.

Figure 2. Geometric illustration of a sample repre-senting a mesio-occlusodistal inlay. a. The occlusalview is shown; the arrow indicates grinding direc-tion. b. The proximal transverse cross section, asindicated by the single asterisk in a, is shown. c.The mesiodistal central cut, as indicated by thedouble asterisk in a, is shown. B: midproximal; H:central-occlusal; E: gingival/axial line angles; and F:central floor.

The objective of this studywas to examine the grindingprecision and accuracy of fit ofinlays in mesio-occlusodistalpreparations produced by aCEREC 2 CAD-CIM unit.

MATERIALS ANDMETHODS

To study grinding precision, weused a CEREC 1 (c) unitequipped with an electric driveand a CEREC 2 unit (Figure 1).For the CEREC 1 unit, we usedthe COS 2.1 software. TheCEREC 2 unit ran on speciallydeveloped software (COS 4.01 13;

Siemens AG). We also usedgrinding disks with 64-,um-di-ameter diamonds in the coatingand ceramic blocks, size I 10 (10x 9 x 15 millimeters), of thefollowing materials:- Vita CEREC Mark II (VitaZahnfabrik);- Dicor MGC glass ceramiclight (Dentsply Int.).

The software used in bothCEREC units contained thespecial design of a geometricallystylized mesio-occlusodistalinlay of known dimensions(Figure 2). However, becausesoftware engineering time was

unavailable, the programs usedfor the samples machined bythe CEREC 1 and CEREC 2units could not be modified toproduce samples of identical di-mensions. Since direct compari-son was not possible, we chosedimensional variation as an in-dependent parameter for the ex-amination of grinding precision.

Production of geometricsamples. Before starting, wemounted new grinding disksand recalibrated the grindingunits by activating the auto-matic calibration mode that isincluded in every software ver-

48 JADA, Vol. 128, January 1997

RESEARCH

sion. The grinding disks werechanged in both units after thesame number of grinding opera-tions and after the message"please replace the wheel" wasdisplayed on the screen of theCEREC 1 unit. After three sam-ples were milled, we replacedthe cooling water with new tapwater, to which 14 milliliters ofcooling lubricant (CERECCimplus D14-A) was added. Forthe grinding process, the geo-metric samples were dividedinto four groups:- Vita Mark II with CEREC 1(n = 40);- Dicor MGC with CEREC 1(n = 40);- Vita Mark II with CEREC 2(n = 40);- Dicor MGC with CEREC 2(n = 40).

Thus, 40 samples of each ofthe two materials were producedby the test unit (CEREC 2) andthe control unit (CEREC 1) for atotal of 160 samples.

Assessing grinding preci-sion. To evaluate grinding preci-sion, we measured four lineardimensions on each of the stand-ardized geometric samples, asshown in Figure 2. The locationsof the linear measurements wereselected and defined according tothe action ofthe grinding wheel.Measurements B (midproximal)and H (central-occlusal) relatedto the grinding action ofthe radi-al cut. Measurements E (gingi-val/axial line angles) and F (cen-tral floor) related to the grindingaction that was oriented to thelong axis ofthe sample.

The samples were fixed onstandard SEM holders andmounted in the 3-D scanningmeasuring device on a three-coordinate table.14 We used amicroscope with an internal ret-icle14 to position the samples insuch a way that the starting

and end points of the respectivelinear measurement on thesample had the same focal dis-tance as each other in the mi-croscope and coincided with theX-axis of the coordinate table.

Beginning with the startingpoint centered under the reticle,we then moved the three-coord-inate table straight toward theend point, using computer-con-trolled 100-jim steps first and1-pm steps for the final 10-jimapproach. The overall distance(in micrometers) was thenrecorded on the screen.14

Statistical analysis ofgrinding precision. Differ-ences in the amount of variationin dimensional data betweenCEREC 1 and CEREC 2, as as-sessed by measurements B, H,E and F, were examined usingthe F test, including correctionfor multiple test situations ac-cording to Bonferroni."5Standard deviation was used toexpress variation of the data,and P-values for statistical dif-

ferences are presented in Table1. In addition, all B, H, E and Fdata for VITA Mark II andDicor MGC were pooled forCEREC 1 (n = 320) and CEREC2 (n = 320), and standard devia-tions were compared.Accuracy of fit of inlays in

mesio-occlusodistal prepara-tions. Twelve extracted humancaries-free molar teeth were pre-pared with a mesio-occlusodistalClass II cavity. We prepared onegingival margin to approximate-ly 1.5 mm above the cementoe-namel junction and called thisthe "deep" preparation. We pre-pared the other gingival marginto include the cementoenameljunction and called this the "verydeep" preparation.16"7 Six teethwere restored with individual in-lays generated by the CEREC 2unit, and the other six teethwere restored with individual in-lays generated by the CEREC 1unit.7"18

For the CEREC 2-generatedinlays, we selected the extrapola-

JADA, Vol. 128, January 1997 49

IESEAR CH

Figure 4. Manufacturing a mesio-occlusodistal testinlay is a fully automatic process. Synchronous ac-tions of the grinding wheel (floor, walls, occlusion)and bur (occlusion) are shown.

Figure 3. The completed design of adistal test inlay on the CEREC 2 monthe computer-generated occlusal suithe central main fissure and cusp sic

tion design mode that automati-cally generates an individualizedocclusal-surface morphology."Figure 3 illustrates a typical fin-ished extrapolation design of atest inlay, and Figure 4 showshow the inlay is machined.

Because of similar results re-garding grinding precision forthe two materials, we chose touse only one material (VitaMark II porcelain) for the ma-chining of individual inlays inthe accuracy-of-fit evaluation.We used Brilliant Lux microfinehybrid (Coltene/Whaledent) asthe luting material and A.R.T.Bond (Coltene/Whaledent) asthe adhesive for the inlays.Working without waterspray,

we seated theinlays usingultrasonic in-sertion21 withthe help of anultrasonicscaler toppedwith a spheri-cal plasticplug. The

mesio-occluso- replica tech-itor displays nique used forrface, includingipes. identical repro-

duction of therestored teeth,

as needed for the SEM exami-nation, was carried out as de-scribed above. Under a scan-ning electron microscope, wemeasured the widths of themarginal interfaces of all inlaysin 36 locations: 12 occlusal, fourshoulder, four proximal-lateral(deep), four proximal-lateral(very deep), four line angles(deep), four line angles (verydeep), two gingival (deep) andtwo gingival (very deep).922

Statistical analysis of in-terfacial width of inlay mar-gins. The mean widths of themarginal luting material inter-face between the tooth and therestoration of eight margin sec-tors (that is, occlusal; shoulder;

proximal-lateral of deep andvery deep proximal prepara-tions; line angles of deep andvery deep proximal prepara-tions; and gingival margins ofdeep and very deep proximalpreparations) were calculatedby pooling the data for theabove-mentioned margin loca-tions. We used the t-test to dis-cover any significant differencesbetween CEREC 1 and CEREC2, correcting for multiple testsituations according toBonferroni.

Photographic and SEMdocumentation of inlays. Wephotographed all restored teethfrom the occlusal, mesial anddistal aspects using a cameraequipped with a 200-mm lenswith x2 magnification. To docu-ment luting interfaces, SEMpictures were taken at xlO0magnification (Figure 5).

RESULTS

Grinding precision. For boththe Vita Mark II and DicorMGC restoration materials,standard deviations in the B di-mension (midproximal) weresignificantly lower (P < .05)with CEREC 2 than withCEREC 1 (Table 1). In the H di-

50 JADA, Vol. 128, Januar-y 1997

RESEARECI

Fi.r._.- A.; ThcIaI~ t ta _ m' _.'.g.n.- ..'.,,.',v.r,.1.n,X,t_w782-

Framm 4-Inly: la hewit. 5 OESRC I(fptos AMmargin (a~autnlhg electrorn_ ny X100), witha(aE" kting Intal wit C.l ONEC margin(SEM~ XIQO), with a qonstant _mafl Interlal widh. ~inlays and 84±t

mension (central-occlusal), sig-nificant differences betweenCEREC 1 and CEREC 2 sam-ples were also found with VitaMark II (P < .001) and DicorMGC (P < .01). In addition,standard deviations for dimen-sions E (gingivalaxial line an-gles) and.F (central floor)showed significt differencesbetween the CEREC 1 andCEREC 2 units for both restora-tion materials exami'ned.Standard deviations of pooledCEREC 2 measurements weresmaUler than those ofCEREC 1measurements by a;factor of 2.4.Accuracy of it. The interfa-

cial width ofthe CEREG 1 andCEREC 2 inlays differed signifi-cantly in margin sections at thedeep and very deep line anglesof proximal preparations as wellas at the gingival margins ofdeep proximal preparations(P < .05) (Table 2). There wasa strong and constant tendencyof the CEREC 2 inlays to havesmaller interfacial widths thanthose ofthe CEREC 1 inlays.When data for all 36 margin lo-cations were pooled, the meaninterfacial widths were56 ±- 27 pm for CEREC 2

38 pm forCEREC 1 inlays, a statisticallyinsignificant difference(P > .05). Typical SEM find-ings of occlusal margn sectionsare shown in Figure 5.

DISCUSSION

Grinding precision ofCAD-CIM-machined restorations is acrucial prerequisite to high ac-curacy of flt.'2 In this study,we used the standard deviation

I _e 'd,

amM-.t. .-

of defined linear measurementson geometric inlay samples ma-chined from Vita CEREC MarkII and Dicor MGC ceramics toeaiegrinin preciLsion.

This approach corresponded-to aprevious evaluation ofthe grind-ing precision of a CEREC unit.4.A small standard deviation im-plies high grinding precision inthe machining ofthe samples.Irrespective ofthe materialused, the grinding precision ofthe-CEREC 2 unit was 2.4 timeshigher than that ofthe CEREC1 unit, as determined by theratio of the standard deviationfor CEREC 1 pooled linear mea-surements to the standard devi-ation- for CEREC 2 pooled linearmeasurements.

Grinding precision generallyis limited by the size and shapeofthe grinding tools and toolcontrol.13 Feed rate, depth of cut,grain size ofthe abrasive materi-al (diamonds) on the tools, watercooling system and chemical ad-ditives to the coolant water arefactors influencing surface quali-ty and dimensions of machinedceramic workpieces.711,225 Theimproved grnding precision ofCEREC 2 vs. CEREC 1 as ob-served in this study can be at-tributed to the generalimprovement in electronics andengineering know-how accumu-lated during the developmentalstages ofCEREC 1.1'

Variations in mesiodistal

JADA, Vol. 128, January 1997 51

HESEARCH-

Lime angle___0s-;_ r0(deep) 124 + 44 59 ± 30 <.05

(verydeep) 109±55 67±27 <.05

Gingival(deep) 8 1 32 31 18 c .05

(very er:tep) 0 ; 0 50 ;7 i W4f02

All margin sections 84 ± 38 56 + 27 NS

Bonfer 5

sample dimensions E and F ob-viously were influenced by wearand deformation of the grindingwheels. Loss of cutting abilityduring the service life of a grind-ing wheel causes increased loadbuildup between the tool and ce-ramic blank, resulting in defor-mation of the tool.11 CEREC 2was less affected by this phe-nomenon. The loss of contour fi-delity, as observed in CEREC 1samples, as well as the higherstandard deviation of the overallinterfacial width of margins inCEREC 1 inlays can also be at-tributed in part to this effect.

Automatic calibration of thetools before each restoration isground has been optimized withCEREC 2 and has contributed tothe improved grinding precisionand accuracy of fit of the restora-tions.

The results of this study sup-port laboratory13 and clinical26experience showing that thefirst-fit rate ofCEREC 2 restora-tions is high. Improvements inthe 3-D camera contributed con-siderably to the increased accu-racy of fit of CEREC 2 inlays.The CEREC 2 camera displaysthe tooth preparation with x12magnification on the high-reso-lution color monitor, whichshows more detail than both theCEREC 1 monitor (x8 magnifica-tion) and magnifying glasses (x2to x4 magnification).'9

The computer-aided design ofCEREC 2 restorations has beensimplified by clear programsteps for the user to follow aswell as automatic design stepsthat prevent operating errors toa large extent.

In our study, the overall mean

interfacial width of CEREC 2 in-lays (56 ± 27 pm) was smallerthan that of CEREC 1 inlays (84± 38 gm); it also was smallerthan the interfacial widths ofCEREC 1 inlays reported inother studies.58912 Differences inour study were statistically sig-nificant (P < .05) at line anglesin deep and very deep proximalpreparations as well as at deepgingival margins. In a recentstudy, the mean occlusal interfa-cial width of CEREC 2 inlayswas 48 ± 34 pm, representing a35 percent improvement overCEREC 1 inlays.27 The CEREC 2interfacial widths reported inthis study tended to be smallerthan those achieved with othermachining systems. 12,28-33

Despite favorable clinicallong-term results of CERECrestorations,5'34 38 researchershave assumed that interfacialwidth is the limiting factor withregard to long-term clinical suc-cess of adhesively placed ceramicrestorations, thus requiring ahigher accuracy of fit.4'39 A clini-cal investigation of aesthetic in-lays measured the wear rate ofluting agents and found thatmost of the wear occurred duringthe first 12 months of clinicalservice; it then tended to levelout.8 A recent analysis of clinicalstudies of CEREC restorationssupported these findings.40

Holmes found that excellentfull-coverage castings typicallyhave a gap of about 40 to 60 pmbeween the margin ofthe prepa-ration and the margin of therestoration.41 The accuracy of fitofCEREC 2 restorations in ourstudy (mean interfacial width,56 ± 27 ,m) and in a recentstudy (48 ± 34 pm)27 is close tothis standard of good precision.In addition, the SEM survey inour study showed highly con-stant interfacial width and con-

52 JADA, Vol. 128, January 1997

B ESEAR C H

tinuous adhesion of the inlays,thus fulfilling an important clin-ical requirement.4 High-vscositycementation using light-cured, mi-crofine, hybrid composite20 and ul-trasonic insertion2' proved to bevery practical in this study and isa standard procedure in our clinic.

CONCLUSION

The improvements in CAD-CIMtechnology, as represented bythe CEREC 2 unit, provide den-tal practitioners and dental lab-oratory technicians with a fastand easy production method forceramic restorations using in-dustrially prefabricated VitaMark II and Dicor MGCmachinable ceramic materials.As a result of high machiningprecision, we found an excellentaccuracy of fit (56 + 27 gm) inthis study. The small standarddeviation indicates the veryhigh reliability of CAD-CIMproduction. Experience in ourclinic shows that the same de-gree of accuracy of fit is true forother CEREC 2 restorations,such as onlays, veneer lami-nates and single full crowns.

Dr. Mormann is one of the original develop-ers of the CEREC system. He is president ofthe board of the Foundation for Computer-

Assisted Dentistry inZZurich, Switzerland,a nonprofit organiza-tion tha receives roy-

alties on the sale of

the CEREC system.

Dr. Mormann Is a

professor and direc-

tor, Division of Aes-

thetic and ComputerRestorations, Clinicof Preventive Den-

tistry, Perlodontol-ogy and Carlology,University of Zurich,Dental School, Plat-tenstrasse 11,CH-8028 Zurich,Switzerland. Address

reprint requests toDr. Mormann.

Dr. Schug is a lectur-or, Division of Aes-

thetic and ComputerRestorations, Clinic

of Preventive Den-

tistry, Peruodontol-ogy and Cariology,Dental School,University of Zurich,Switzerland.

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